24 research outputs found
Manipulation of Stable Matchings using Minimal Blacklists
Gale and Sotomayor (1985) have shown that in the Gale-Shapley matching
algorithm (1962), the proposed-to side W (referred to as women there) can
strategically force the W-optimal stable matching as the M-optimal one by
truncating their preference lists, each woman possibly blacklisting all but one
man. As Gusfield and Irving have already noted in 1989, no results are known
regarding achieving this feat by means other than such preference-list
truncation, i.e. by also permuting preference lists.
We answer Gusfield and Irving's open question by providing tight upper bounds
on the amount of blacklists and their combined size, that are required by the
women to force a given matching as the M-optimal stable matching, or, more
generally, as the unique stable matching. Our results show that the coalition
of all women can strategically force any matching as the unique stable
matching, using preference lists in which at most half of the women have
nonempty blacklists, and in which the average blacklist size is less than 1.
This allows the women to manipulate the market in a manner that is far more
inconspicuous, in a sense, than previously realized. When there are less women
than men, we show that in the absence of blacklists for men, the women can
force any matching as the unique stable matching without blacklisting anyone,
while when there are more women than men, each to-be-unmatched woman may have
to blacklist as many as all men. Together, these results shed light on the
question of how much, if at all, do given preferences for one side a priori
impose limitations on the set of stable matchings under various conditions. All
of the results in this paper are constructive, providing efficient algorithms
for calculating the desired strategies.Comment: Hebrew University of Jerusalem Center for the Study of Rationality
discussion paper 64
A Hydraulic Approach to Equilibria of Resource Selection Games
Drawing intuition from a (physical) hydraulic system, we present a novel
framework, constructively showing the existence of a strong Nash equilibrium in
resource selection games (i.e., asymmetric singleton congestion games) with
nonatomic players, the coincidence of strong equilibria and Nash equilibria in
such games, and the uniqueness of the cost of each given resource across all
Nash equilibria. Our proofs allow for explicit calculation of Nash equilibrium
and for explicit and direct calculation of the resulting (unique) costs of
resources, and do not hinge on any fixed-point theorem, on the Minimax theorem
or any equivalent result, on linear programming, or on the existence of a
potential (though our analysis does provide powerful insights into the
potential, via a natural concrete physical interpretation). A generalization of
resource selection games, called resource selection games with I.D.-dependent
weighting, is defined, and the results are extended to this family, showing the
existence of strong equilibria, and showing that while resource costs are no
longer unique across Nash equilibria in games of this family, they are
nonetheless unique across all strong Nash equilibria, drawing a novel
fundamental connection between group deviation and I.D.-congestion. A natural
application of the resulting machinery to a large class of
constraint-satisfaction problems is also described.Comment: Hebrew University of Jerusalem Center for the Study of Rationality
discussion paper 67
A Stable Marriage Requires Communication
The Gale-Shapley algorithm for the Stable Marriage Problem is known to take
steps to find a stable marriage in the worst case, but only
steps in the average case (with women and men). In
1976, Knuth asked whether the worst-case running time can be improved in a
model of computation that does not require sequential access to the whole
input. A partial negative answer was given by Ng and Hirschberg, who showed
that queries are required in a model that allows certain natural
random-access queries to the participants' preferences. A significantly more
general - albeit slightly weaker - lower bound follows from Segal's general
analysis of communication complexity, namely that Boolean queries
are required in order to find a stable marriage, regardless of the set of
allowed Boolean queries.
Using a reduction to the communication complexity of the disjointness
problem, we give a far simpler, yet significantly more powerful argument
showing that Boolean queries of any type are indeed required for
finding a stable - or even an approximately stable - marriage. Notably, unlike
Segal's lower bound, our lower bound generalizes also to (A) randomized
algorithms, (B) allowing arbitrary separate preprocessing of the women's
preferences profile and of the men's preferences profile, (C) several variants
of the basic problem, such as whether a given pair is married in every/some
stable marriage, and (D) determining whether a proposed marriage is stable or
far from stable. In order to analyze "approximately stable" marriages, we
introduce the notion of "distance to stability" and provide an efficient
algorithm for its computation